1. Field of the invention
This invention relates to a brushless motor.
2. Related art
FIG. 7 shows an example of a conventional brushless motor.
The brushless motor is of a three-phase type. In FIG. 7, reference numeral 1 designates the rotary shaft of the brushless motor. The rotary shaft 1 is rotatably supported through bearings 4 by a bearing holder 3 which is mounted on a printed circuit board 2. A cup-shaped rotor casing 5 is fixedly mounted on the upper end portion of the rotary shaft 1 as viewed in FIG. 7 so that it is turned together with the rotary shaft 1. An annular rotor magnet 6 is fixedly secured to the inner surface of the cylindrical wall of the cup-shaped rotor casing 5 in such a manner that the lower end face of the rotor magnet 6 is substantially flush with the lower end face of the cylindrical wall of the rotor casing 5 as viewed in FIG. 7, and those lower end faces are close to the upper surface of the printed circuit board 2. The annular rotor magnet 6 is magnetized so that it has N and S poles alternately in the direction of circumference. A stator core 19 having drive coils 9 is provided inside the rotor magnet 6 in such a manner that the stator core former 19 is confronted with the rotor magnet 6.
Three Hall elements 7a, 7b and 7c serving as magnetic detectors are provided so as to be confronted with the magnetized surface of the rotor magnet 6. More specifically, the Hall elements 7a, 7b and 7c are arranged on the printed circuit board 2 at intervals of an electrical angle 120.degree.. A driving IC 8 is mounted on the printed circuit board 2. The driving IC 8 has a drive circuit (not shown) which receives the output signals of the Hall elements 7a, 7b and 7c and applies drive currents to the drive coils 9 according to the output signal, to turn the rotor.
An indexing magnet (a magnet piece) 20 is mounted on the outer surface of the cylindrical wall of the rotor casing 5. An index sensor (such as a magnetic head) 21 is provided, as a magnetic sensor, on the printed circuit board 2 in such a manner that it is confronted with the outer surface of the cylindrical wall of the rotor casing 5. The index sensor 21 detects the magnetic field of the indexing magnet 20, to provide an output signal. The output signal of the index sensor 21 is amplified by an amplifier (not shown) provided on the printed circuit board 2, to form an index signal (such as a pulse signal) whose period corresponds to one revolution of the rotor. The index signal is utilized for detection of the position of the brushless motor.
The conventional brushless motor thus organized suffers from the following difficulties:
It is essential for the motor to have the indexing magnet 20 and the index sensor 21 as position detecting means, which increases the number of components and accordingly the manufacturing cost of the motor.
On the other hand, the index signal (or position detection signal) must be coincident with a predetermined track start position. This means that the position of the index sensor must be precisely adjusted circumferentially of the motor. The adjustment is considerably difficult.